dt-bindings: add device binding for the CCU of Allwinner V3s

Allwinner V3s is now driven by sunxi-ng CCU driver.

Add devicetree binding for it.

Signed-off-by: Icenowy Zheng <icenowy@aosc.xyz>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>